Considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
Typically, hydrogen is produced by a variety of methods such as water electrolysis, steam reforming, cracking hydrocarbons, or cracking ammonia. In all of these cases, the hydrogen that is produced is dried and cleaned of all contaminants and then either compressed at high pressure, liquified, or stored in an alloy as a hydride material. All methods, however, have benefits and disadvantages. Those methods that use fossil fuels as a starting raw material for hydrogen production have a built in disadvantage in that they produce carbon monoxide and carbon dioxide as part of their hydrogen generation which pollute the atmosphere. In addition, fossil fuel availability is limited and the remaining reserves are better used for other industrial chemical use than being used as fuel. Water electrolysis is preferred, because there is an unlimited supply of water and water electrolysis produces no pollution. Although energy intensive, water electrolysis can be performed using electricity from any source such as off peak power, solar power, geothermal power, or wind power. It is also recognized that the purest form of hydrogen is produced via water electrolysis.
Electrolysis is the electrolytic decomposition of water in an electrolyte and has long been practiced for the production of hydrogen gas. Traditionally, alkaline solutions have been used for electrolysis for the production of hydrogen. Hydrogen is also evolved as a byproduct of certain other industrial electrolysis processes such as chlorine production. The major components of the electrochemical cell in which electrolysis takes place include an anode and a cathode which are in contact with an electrolytic solution, and a separator used to separate the anode and cathode and their reaction products. In operation, the selected electrolyte, such as NaOH, KOH, or H2SO4, is continually fed into the cell and a voltage is applied across the anode and cathode. This produces electrochemical reactions which take place at the anode and cathode to form oxygen and hydrogen gas. The overall reactions are represented as:Anode: 2OH−−>½O2+2e−+H2OCathode: 2H2O+2e−−>H2+2OH−Total: H2O−>H2+ 1/2 O2
When hydrogen is produced, it is collected, stored, and transported to end users. Hydrogen may be stored in gas, liquid, or solid (hydride) form. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. Additionally, large and very expensive compressors are required to store hydrogen as a compressed gas, and compressed hydrogen gas is an explosion/fire hazard. Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since liquid hydrogen is extremely dangerous and presents a potential explosion hazard. Liquid hydrogen also must be kept extremely cold, below −253° C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen. Another drawback to storage as a liquid is the costly losses of hydrogen due to evaporation, which can be as high as 5% per day.
In addition to the problems associated with storage of gaseous or liquid hydrogen, there are also problems associated with the transport of hydrogen in such forms. For instance transport of liquid hydrogen will require super-insulated tanks, which will be heavy and bulky and will be susceptible to rupturing and explosion. Also, a portion of the liquid hydrogen will be required to remain in the tanks at all times to avoid heating-up and cooling down of the tank which would incur big thermal losses. As for gaseous hydrogen transportation, pressurized tankers could be used for smaller quantities of hydrogen, but these too will be susceptible to rupturing and explosion. For larger quantities, a whole new hydrogen pipeline transportation system would need to be constructed or the compressor stations, valves and gaskets of the existing pipeline systems for natural gas will have to be adapted and retrofitted to hydrogen use. This assumes, of course, that the construction material of these existing pipelines will be suited to hydrogen transportation. Hydrogen is known to permeate through many materials. In high strength steels, hydrogen induced cracking is common.
Certain metals and alloys have been known to permit reversible storage and release of hydrogen. In this regard, they have been considered as superior hydrogen storage material, due to their high hydrogen storage capacity. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase metal or alloy system can store large amounts of hydrogen by absorbing hydrogen with a high density and by forming a metal hydride under a specific temperature/pressure or electrochemical conditions, and hydrogen can be released by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time, since they are formed by the insertion of hydrogen atoms to the crystal lattice of a metal. A desirable hydrogen storage material must have a high gravimetric and volumetric density, a suitable desorption temperature/pressure, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
Many metal alloys are recognized as having suitability for hydrogen storage in their atomic and crystalline structures as hydride materials. While this storage method holds promise to be ultimately convenient and safe; improvements in efficiency and safety are always welcome. This invention provides such improvement.
While metal hydrides are viewed as a viable way of safely and efficiently storing hydrogen, there is still a need for improved processes for collecting and transporting hydrogen to end users. As hydrogen is produced, it is normally collected, compressed and absorbed into hydrogen storage alloys for storage and transportation. By eliminating some of these steps, an efficient process of producing and transporting hydrogen may be realized which can both decrease cost of hydrogen production while providing for a safe and cost effective means of distributing hydrogen to end users.